Many types of spinal irregularities such as scoliosis, kyphosis, lordosis, etc., can cause pain, limit range of motion, or injure the nervous system within the spinal column. These irregularities can result from trauma, tumor, disc degeneration, and disease. Oftentimes, these irregularities are treated by adjusting a portion of the spinal column and immobilizing that portion of the spine. This treatment involves affixing a plurality of pedicle screw assemblies (which includes a tulip element and a bone fastener) to one or more vertebrae and then connecting the pedicle screw assemblies to an elongate rod that generally extend in the direction of the axis of the spine. The connection is typically achieved by attaching the elongated rod to the tulip element of each pedicle screw assembly affixed to the vertebras.
There are many types of pedicle screw assemblies that surgeons can use to immobilize a portion of the spinal column. Such assemblies include, for example, polyaxial, monoaxial, and uniplanar pedicle screw assemblies. Each of these assemblies is characterized by whether their tulip element can be angulated relative to the bone fastener and in which plane(s) of the bone fastener.
A polyaxial pedicle screw assembly provides the greatest flexibility in tulip movement. More specifically, a tulip element of a polyaxial assembly is allowed to pivot freely in any direction and in all planes relative to a central axis of its respective bone fastener. This feature enables polyaxial assemblies more variability in rod placement.
A monoaxial pedicle screw assembly typically has a unitary construction in that the bone fastener and tulip element are machined from a single piece of biocompatible metal or plastic. This type of construction prevents the monoaxial pedicle screw assembly from being able to pivot about the bone fastener. This type of unitary construction also disallows the tulip element from being uncoupled from the bone fastener.
A uniplanar pedicle screw assembly, on the other hand, allows its tulip element to pivot back and forth in one plane about a central axis of the bone fastener, while simultaneously preventing movement of the tulip element in all other planes. This feature enables uniplanar assemblies to maintain rigidity in a particular plane during deformity correction. It should be noted, however, that uniplanar assemblies share a common property with monoaxial assemblies; that is, the tulip element of a uniplanar assembly cannot be uncoupled from the bone fastener and requires a different bone fastener head geometry to provide rigidity. This is due to the fact that the bone fastener head of a uniplanar assembly has a geometry that is different from the bone fastener head of a polyaxial assembly. The geometry for a uniplanar assembly is designed to resist angulation in one direction, but it also requires a more complex assembly process that is time consuming and dangerous in-situ.
With this knowledge, surgeons performing surgery to correct spinal irregularities will typically decide beforehand on which of the pedicle screw assemblies to affix to a pedicle of a particular vertebra. For example, a surgeon may decide beforehand to use a polyaxial pedicle screw assembly on a particular vertebral body for ease of rod placement. Based on this decision, the surgeon will affix the polyaxial assembly (which includes a tulip element and a bone fastener) as a single unit onto that vertebral body. However, after affixing the polyaxial assembly onto the vertebral body, the surgeon may determine intra-operatively that deformity correction is needed at the affected vertebral body and that a uniplanar assembly would be more suitable for this purpose.
One way to solve this problem is to replace (e.g., by unscrewing) the polyaxial assembly initially affixed to the vertebral body, and then affixing (e.g., by screwing) a uniplanar assembly into the same hole. However, this solution is time consuming because it requires multiple steps and may compromise the structural integrity of the pedicle/vertebra due to the repeated screwing and unscrewing of bone fasteners.
Because of these deficiencies in the prior art, there exists a need to provide a modular pedicle screw assembly that enables a surgeon to intra-operatively use different types of tulip assemblies without having to screw and unscrew bone fasteners.